In recent years, the research and development of high density optical disk systems that can carry out recording/reproduction of information using blue-violet color semiconductor laser of wavelengths with about 400 nm has been progressing rapidly. As one example, in an optical disk carrying out information recording and reproduction with the specifications of an NA of 0.85, and light source wavelength with 405 nm, that is, in a so-called Blu-ray Disc (BD), for an optical disk of 12 cm diameter which is the same size as that of a DVD (NA of 0.6, light source wavelength with 650 nm), it is possible to record 20 to 30 GB of information per side, and further, in an optical disk carrying out information recording and reproduction with the specifications of an NA of 0.65, and light source wavelength with 405 nm, that is, in a so-called HD DVD, for an optical disk of 12 cm diameter, it is possible to record 15 to 20 GB of information per side. In the following, in the present patent description, these types of optical disks are called “High density DVDs”.
However, it cannot be said that the value as a product of the optical pickup apparatus is sufficient when merely appropriate information can be recorded and reproduced in a high density DVD of the above types. At present, considering the current reality that DVDs and CDs with various types of information recorded in them are being sold, it is not sufficient to record and reproduce appropriate information in high density DVDs, and, for example, the ability to record and reproduce appropriate information in a similar manner in the conventional DVDs or CDs possessed by the user leads to increasing the value of the product as a compatible type of optical pickup apparatus. Because of this background, the optical system used in a compatible type optical pickup apparatus, of course must be low in cost and have a simple configuration, and in addition, it is desirable that it should be possible to obtain a good spot size for recording and reproducing appropriate information in high density DVDs, and conventional DVDs and CDs also. Further, although optical pickup apparatuses have been realized that can record and/or reproduce information in a compatible manner for DVDs and CDs, there is the current situation that further size reduction, thickness reduction, and cost reduction on the current configuration are being desired.
However, in order to realize an optical pickup apparatus having compatibility with DVDs and CDs, objective lenses provided with diffraction structures have been developed. As such an objective lens, for example, there is one in which, on one surface of the objective lens, different diffraction structures are provided inside and outside a prescribed distance h from the optical axis in a direction perpendicular to the optical axis, in the inner region, spherical aberration is corrected for the different protective substrate thicknesses, and in the outer region, spherical aberration is corrected only at the time of using a DVD but flaring is done without correcting the spherical aberration at the time of using a CD. By forming an objective lens of this type, it becomes possible to form appropriate focusing spots required at the time of recording or reproducing the respective information in the different optical information recording media (see Patent Document 1).
Patent Document 1: Unexamined Japanese Patent Application Publication No. 2004-171709.
It is also possible to form this type of diffraction structure, not limited to on the objective lens, but on the collimator lens or coupling lens, or in a dedicated optical element. In addition, on the one hand, in order to absorb the aberration error (color aberration) due to variations in the wavelength of the laser light source or wavelength fluctuations, there are cases in which a diffraction element is provided, of course, in the light emission system from the light source to the optical disk.
However, when a diffraction element is provided in the light emission system as in these examples, there was the problem that, because of variations in the diffraction efficiency, there were variations in the amount of light focused on the information recording surface of the optical disk. In other words, the temperature of the diffraction element changes along with changes in the ambient temperature, the diffraction efficiency of the diffraction element used in the light emission system changes, or else the temperature of the light source changes or the amount of light emitted changes, the wavelength of the emitted light changes, and as a result, very often the diffraction efficiency of that diffraction element changes.
FIG. 5 is a simulation diagram showing the state of the spot of light focused on the recording surface of an optical disk and the flare, and in this case, the focused light spot SPT is at the center point, and surrounding it is the flare. The flare shown here is generated when the temperature of the diffraction element DE formed on the optical surface of the objective lens OBJ is different from the design temperature, or when the wavelength of the light flux passing through it is different from the design wavelength. For example, if the design temperature of the diffraction element DE is 25° C. but it is used at 60° C., or if the design wavelength of that diffraction element DE is 405 nm but a light with wavelength 407 nm is incident on it, etc., a part of the light flux passing through the diffraction element DE does not form the focused light spot SPT, but becomes the flare dispersed around the focused light spot SPT. This flare is created by a diffracted light different from the design order of the diffraction element DE. In other words, although the light of different diffraction orders generated by the diffraction element DE interfere with each other and energy is given only to light of a specific order (for example, 10th order) thereby forming the focused light spot SPT by this light of a particular order, when the temperature or passing light wavelength of the diffraction element DE are different from the design conditions, the flare is generated because a part of the diffracted light of orders different from the above specific order (diffracted light of different orders) are not erased completely by diffraction. In this manner, when the temperature of the diffraction element DE or the wavelength of the light passing through it are different from the design conditions, a part of the incident light flux will not be used for the focused light spot SPT that is used for reproduction or recording of information of the optical disk, that is, a loss occurs in the amount of light.
FIG. 1(a) is a diagram showing an example of the relationship between the temperature of the diffraction element and the diffraction efficiency, and FIG. 1(b) is a diagram showing an example of the relationship between the wavelength fluctuations and the diffraction efficiency. In FIG. 1(a), while the diffraction efficiency of the optical source wavelength λH of, for example, a blue-violet laser becomes a maximum around a temperature of 25° C. of the diffraction element, the diffraction efficiency of the optical source wavelength λD of red color laser becomes a maximum around a temperature of the diffraction element of 50° C. In other words, at a temperature of the diffraction element other than the temperature at which the diffraction efficiency becomes a maximum, in the case of the light fluxes of either of the two wavelengths λH and λD, the diffraction efficiency changes in accordance with changes in the temperature. On the other hand, as is shown in FIG. 1(b), the diffraction efficiency changes even when the light source wavelength is shifted from the design wavelength.
FIG. 2(a) is a drawing showing the light flux focused on the information recording surface of the optical disk OD by the diffraction element DE as viewed in a direction at right angles to the optical axis, and FIG. 2(b) is a diagram showing the light flux focused on the information recording surface of the optical disk OD as viewed in a direction at right angles to the optical axis, and while the focused light spot SPT here has a diameter of 0.5 to 1.0μm, surrounding it is flare with a low light amount.
The diffraction efficiency when the light flux is focused on the information recording surface of the optical disk OD through the diffraction element DE of the light emission system shown in FIG. 2(a) is expressed by q/p where p is the energy of the light flux incident on the diffraction surface of the diffraction element DE and q is the energy of the light flux with a specific diffraction order in the spot SPT focused on the information recording surface of the optical disk among the light flux diffracted by that diffraction element DE. However, the diffraction order of 0 is also included.
The diffraction efficiency of the diffraction element is determined by the degree of interference between the light emitted from neighboring diffraction ring bands, and the diffraction efficiency becomes high when the interference is strong, that is, when the phase of the light is the same. In an ordinary diffraction lens, the step between the ring bands is determined assuming the oscillation wavelength of the semiconductor laser used in optical pickup apparatus and the temperature of the diffraction element, so that the phase is the same in those conditions.
However, in actuality, the oscillation wavelength of the semiconductor laser differs depending on the individual laser, or the wavelength of oscillations changes or the diffraction element itself expands or contracts due to changes in the environmental temperature. As a result, because the phase of the light emitted from neighboring ring bands gets shifted, and because the diffraction efficiency changes as is shown in FIG. 1, the amount of light of the light flux with the order of diffraction focused on the recording surface of the optical disk changes.
Since the amount of shift in the phase becomes large as the step between the ring bands designed originally becomes large, particularly in a diffraction structure using diffracted light of a high order of diffraction, the diffraction efficiency changes sensitively with respect to changes in the wavelength and temperature.
For example, when the amount of light in the emitted light from a semiconductor laser changes, because of the so called APC (Auto Power Control) drive control, by guiding a part of the light flux emitted from the semiconductor laser to a monitor element, the amount of emitted light is monitored accurately, and based on the result of that monitoring, it is possible to stabilize the amount of light emitted by the semiconductor laser. However, when the diffraction efficiency changes, since this is not reflected in the result of monitoring, light flux with changed amount of light gets focused as it is on the information recording surface of the optical disk. In that case, if the amount of light focused on the recording surface of the optical disk fluctuates, it is likely that a problem occurs such as either it becoming difficult to carry out good reading of the recorded information or it being not possible to carry out good recording.
In an example of an optical pickup apparatus shown in FIG. 6, the signal from the temperature detector TD that measures the environmental temperature of the optical pickup apparatus is input to the first Auto Power Control APC1 and the second Auto Power Control APC2. When the environmental temperature changes with respect to the set value, based on the graph in FIG. 1(a), the first Auto Power Control APC1 or the second Auto Power Control APC2 carries out laser control in the direction of compensating the change in the diffraction efficiency (for example, increasing the laser power if the diffraction efficiency decreases, or decreasing the laser power if the diffraction efficiency increases). However, it is desirable to place the temperature detector TD in the neighborhood of the actuator of the objective lens OBJ, inside the optical housing, or in its neighborhood.
In this manner, it is also possible to think of, for example, measuring using a thermistor, etc., the environmental temperature or the temperature of the diffraction element of the optical pickup apparatus or in its surroundings, and to change the output of the semiconductor laser in accordance with the result of that measurement. In the example of an optical pickup apparatus shown in FIG. 6, although there is the advantage that it is possible to correct the changes in the diffraction efficiency due to changes in the temperature with a relatively simple and inexpensive configuration, as has been explained above, changes in the diffraction efficiency occur not only due to changes in the environmental temperature but also due to changes in the wavelength of oscillations of the light source, even if only changes in the environmental temperature are measured, it is difficult to adjust the output of the semiconductor laser accurately based on that.